As one of information recording media adapted to be mounted in information storage devices, there is known a magnetic disk adapted to be mounted in a hard disk (HDD). In recent years, it is strongly required to improve the recording capacity of the magnetic disk and thus it is of urgent necessity to increase the recording density thereof and to extend the recording area thereof.
As a factor for enabling high-density recording, it is necessary to reduce as much as possible the flying height of a magnetic head with respect to the magnetic disk and, for that purpose, it is necessary to make smoother a surface of the magnetic disk.
In order to extend the recording area, it is necessary to ensure as large as possible a smooth region of the main surface. However, depending on polishing conditions of a glass substrate, a surface-down in which a surface is lowered relative to a main surface of the glass substrate or a surface-up (hereinafter referred to as a rise) relative to the main surface occurs at an outer peripheral end portion of the substrate. When the magnetic head flies over the magnetic disk having such a shape, the head may be inclined at the surface-down or rise portion to make its flight unstable, thus causing a crash. The surface-down or rise portion has been impeding the extension of the recording area.
In recent years, there have been actively developed a magnetic head attached with a pad for preventing adhesion of the magnetic head even if a magnetic disk is smooth and a LUL (Load/Unload) system that enables realization of a lower flying height. Normally, in the case of this LUL system, a surface of a magnetic disk is smooth and, while the magnetic disk is stopped, a magnetic head stands by on the outside of the magnetic disk and, after the magnetic disk starts rotation, the magnetic head moves from the outside of the disk and flies over the surface of the disk to perform recording/reproduction. Therefore, in general, a lower flight is achieved as compared with the CSS system. In the case of the LUL system, in order to ensure the flying stability of the magnetic head, it is necessary to control an outer peripheral end portion shape of a substrate more strictly than in the case of the CSS system. In the case of the LUL system, since the low flight of the head is enabled, higher-density recording is made possible as compared with the CSS system.
In view of this, in order to realize a low flight of a magnetic head in the LUL system, there is an invention that achieves a smoothness high enough for enabling high-density recording and specifies an outer peripheral end portion shape to a predetermined value (e.g. specifying a surface-down in which a surface is lowered relative to a main surface of a glass substrate, or the like) for enabling extension of a recording area to the periphery (JP-A-2004-265582 (Patent Document 1)).    Patent Document 1: JP-A-2004-265582